texts: Retrofitting a DSLR camera to a super-cheap Tasco reflector telescope

texts:
I was given a 1995 vintage Tasco "302003" Newtonian telescope recently and have had some exciting success with it.  I have managed to view the Moon in beautiful detail and also had my first glimpse of Jupiter (with Enceladus Ganymede & Europa in the same shot!).

I was able (with very considerable difficulty) to photograph both Moon and Jupiter scenes by holding my camera up to the eyepiece.  Far from ideal but it has really fired my imagination about doing more astronomical photography.

Here's a picture of my telescope and it's accessories.  Despite spending the last 15 years in the back room of a charity shop, it's in brand new condition and all the parts are present including the tripod and all fixings!...

(Blue arrows indicate places where eyepiece lenses can be inserted).

My camera is a Nikon D40 (APS-C format, DX lens ring system).

Is it possible to modify this telescope's eyepiece (a rack-and-pinion tube-length adjustable type) so that I can attach my camera in order to take photographs of astronomical subjects?  Photographs of the Moon would be my primary goal.

I am happy to make severe modifications to the telescope if necessary.  But I realise that certain measurements would be absolutely critical to making good photographs possible.  I think I can make an adaptor that goes between the eyepiece socket and the camera.  But I don't know all of the parameters I will have to know to design such an adaptor.

Problems I believe I need to solve...

I think that holding the camera's central axis perfectly concentric with the eyepiece's axis is critical.
I need some sort of Relay Lens between the secondary mirror and the camera body.
Keeping the rack-and-pinion tube adjustment (actually a fairly sturdy mechanism) is important and would allow better focusing of the picture as the camera is not likely to have enough auto-focus range to cope with the 700mm F length.

I do have a small Milling machine and most of the basic machine tools.  I have a ready supply of aluminium stock and industrial plastic stock (RG1000) that I can build an adaptor from.

Is this project feasible?

I'm not looking for perfect photos, but rather something that is better and more convenient than holding my cellphone camera close to the eyepiece and holding my breath! :)

My astronomy/optics knowledge is practically zero but I'm a fairly savvy mechanical engineer and can build things to a specification.

Thank you for reading, I look forward to your responses.


texts: How do you convert an observed magnitude to a flux?

texts:
For a series of supernovae I have the observed magnitude and I want to convert these values to flux.

I understand that I need to apply:

$m - M_r = -2.5\log_{10} \frac{F}{F_r}$

where $M_r$ is a 'reference magnitude' with corresponding known 'reference flux'.

What exactly should I use as my reference values?


texts: When a moon is in orbit around it's planet but is so close that it will become tidally locked does it experience a large 'wobble' in between the phase when it still has its own rotation and the final phase when it is always showing the same face to the planet?  I can only imagine that it would 'wobble' very widely, back and forth like a pendulum until finally settling into a locked position.


texts: What are the criteria for the classification of galaxies?

texts: There are lots of galaxies, for example, the Milky Way and so on. These galaxies consist of lots of stars. I want to know how galaxies are classified. Is it only by their shape that is caused by gravity? Or are there some other criteria?


texts: What could be the utmost lowest temperature in the universe/multiverse?


texts: Will time that moon crosses meridian always be periodic?

texts: If I observe the moon pass my celestial meridian at a specific north polar clock 'time' will I observe every subsequent passage at ( 'current time' + 13.2* X 'days passed' mod 360* ) ? Does the moon pass celestial meridian at exactly the same time if observed from every point on this longitude?


texts: What is the difference between LMC and SMC?

texts:
Apart from the obvious difference as suggested by their names that the Large Magellanic Cloud is 'larger' than the Small Magellanic Cloud, what are the other differences between them?

Do give all the details you can. Thank you!


texts: Connection between Earth, Moon, Theia, and asteroid belt?

texts:
Are there other fragments aside from the Moon that resulted from the collision of Earth and "Theia" (as it's called in recent main-stream press stories)?  And if not, why not?  And what happened to "Theia" (like were did it go?)

Has there been any consideration or hypothesizing that Earth may have had a larger orbit (say between Mars/Jupiter) until a large body (Theia) bashed into it, sending Earth closer to the sun along with its new satellite, while also creating the asteroid-belt?  Could something like Ceres be the missing "Theia"?

I know it sounds a bit far-fetched, but then so does the whole concept of the Moon being born from a planet collision with Earth (with very little evidence to back it up, at least till recently).


texts: How does glass affect taking solar spectra?

texts: In this case I'm using a CCD camera mounted telescope pointed at the clouds to take solar spectra and was wondering how the telescope being pointed at the clouds through a double gazing (two layer) glass window would affect the spectra, if at all?


texts: Is something wrong with my luminosity calculation?

texts: A few million years after a white dwarf forms, its surface temperature reaches $100000\text{K}$, while its radius is $0.01R_\odot$. Would this mean that its luminosity is $\Big(\dfrac{100000}{5778}\Big)^4 \cdot 0.01^2 = 8.972 L_\odot$? If so, why is it that way (I do understand the Stefan-Boltzmann law), and does that mean that all neutron stars are less luminous than white dwarfs?


texts: What are the current observational limits on the existence of Dyson spheres/swarms/rings?

texts: A Dyson sphere/swarm/ring is a hypothetical structure an extra-terrestrial entity would construct to collect a large fraction of its host star's light, and would likely generate a fairly strong infrared signature that could potentially be detected by modern infrared surveys (such as WISE). Do we have any meaningful constraints on the existence of these structures within the Milky Way (or beyond) from these surveys? What sorts of natural phenomena could be confused with such structures, if they existed?


texts: Planets positions (azimuth, altitude) API

texts:
I'm looking for a simple API that will allow me to get positions of planets and the moon for a given date, location and time.
Does anyone know an API like that?
Any suggestions or alternatives would be appreciated.


texts: Does the observed color of the sun change based on the observer's location?

texts: A friend of mine visited Panama and claimed the sun appeared whiter. He's very color conscience and says certain shades of red looked different there. Could this be due to the position of the sun?


texts: Help understanding why these two sub-Hill sphere moon stability limits are so different?

texts:
Comments under this answer to What orbital period would produce one New Moon (and one Full Moon) each year? What other effects would this produce? link to

Stable satellites around extrasolar giant planets which gives aE ~ 0.4895 (1.0000 - 1.0305eP - 0.2738esat). So for circular obits the limit is 0.4895 RHill. This is based on dynamics simulations.
Transit timing effects due to an exomoon (section 3.1 and Appendix B) which states (with $\chi$ as some fraction of the Hill sphere radius:

$$\frac{P_{moon}}{P_{planet}} = \sqrt{ \frac{\chi^3}{3} }$$
This answer cites Exomoon habitability constrained by illumination and tidal heating that links to item 2, and seems to say that this limit results in the conclusion that stability requires the period of the Moon to be no longer than 1/9 (one ninth) that of the planet's period.

The longest possible length of a satellite’s day compatible with Hill stability has been shown to be about Pp/9, Pp being the planet’s orbital period about the star (Kipping 2009a).

I don't understand how the factor of 1/9 arrises here, nor how these two different stability criteria can be brought into alignment.
For example, if I put 0.4895 from the first item into the equation from the 2nd item, I get 1/5 not 1/9, that's a small disagreement considering the topic of stability is a bit mushy, but I do not see at all how 1/9 arrises without using item 1. How did they get 1/9?


texts: What does the active area on a typical CCD based detector look like?

texts: Pixel based detectors, particularly optical CCDs like what was used in the SDSS camera, are ubiquitous in astronomy. Is there any dead area on the detectors? Not the obvious gaps between each individual sensor, but on the sensor itself. That is, does a typical detector have any gaps between the pixels from microscopic wires laid on the front, alternating doping regions in the silicon, or just areas where a photon can hit and the photo-electron is unlikely to be collected? What fraction of the area is dead?


texts: Acceptable errors in photometric measurements

texts:
As the title says, what is considered as an "acceptable" error in a photometric measurement (in magnitudes)?

I know that it will depend a bit on the final use of the data, i.e., the photometric error acceptable for a measurement meant to observe a planetary transit over a star will not be the same as the photometric error acceptable for measuring a light curve of a variable star, but I suppose that there should be some guidelines, or generally accepted values (for example, related to the SNR of the observation).

The point is that I have a ton of observations in several filters for different objects, each observation with its associated error (derived from the observation). And I need to filter a bit the data, to keep only good quality data. I saw here that errors of 0.10 mags are tolerable, but 0.25 mags means that you might also be detecting noise. So, I would like to know, where should I put the tolerance in the error (at least, from one more source different to that link).

Any help would be appreciated!! :D


texts: Metallicity: What are the metallicity values of various galaxies?

texts: What are the various values of metallicity of various known galaxies, like Milky Way, Large Magellanic Cloud, Small Magellanic Cloud, Andromeda and so on?


texts: Okay, we've seen the super hot Wolf-Rayet stars, especially WR 102 and 142, and the "slash stars," many of which are early O (O2-4.5/WN). We know the temperatures of these Wolf Rayet stars. So if we know that, what would their spectral types be in the standard Morgan-Keenan spectral classification (i.e. O2V or B0Ia+), and are there known stars with spectral class earlier than O2?


texts: How massive does a main sequence star need to be to go type 1 supernova?

texts:
We know the mass a white dwarf needs to be. That's well defined by the Chandrasekhar limit, but before a main sequence star turns into a white dwarf it tends to lose a fair bit of its matter in a stellar nebula.

According to this site, the white dwarf that remains is about half the mass of the main sequence star, with larger stars losing a bit more.

So, the question: Is it correct to say that a star with a mass of about three solar masses will eventually go supernova, similar to a type 1 supernova, even when it's not part of a binary system? Has that kind of supernova ever been observed?

Or does something else happen like in the final stages of that star? Does it keeps going though collapse and expand cycles, losing enough mass that when it finally becomes a white dwarf it's below the Chandrasekhar limit in mass?

Mostly, what I've read on supernovae says that type 1 supernovae happen when a white dwarf accretes extra matter and reaches the limit and type 2 supernovae are much larger and require about 8-11 solar masses to generate the iron core which triggers the supernova. What happens with the death of the star between three solar masses and eight solar masses?


texts: Rendering asteroid orientation as seen from Earth

texts:
I am writing a program to render an asteroid as seen from various parts of the solar system.  My goal is to roughly reproduce the image of Cacus according to information on its DAMIT detail page.

Here is my approach:

Create a solar system at JD0=2443568.0.  The axes of the visualization are arranged according to the ecliptic with positive X axis in the direction of the vernal equinox and other axes according to the right-hand rule.  Earth is positioned at t=JD0.  (image)
Place the asteroid at (0, 0, 0) and begin to rotate the object according to the matrix multiplication described by the Description page and Kaasalainen & Torppa 2001:

φ0=0 and t=JD0, so there is no initial Z axis rotation
Latitude β is -63, so rotate 90 - (-63) = 153 degrees on the Y axis (image)
Longitude λ is 251, so rotate the object 251 degrees anticlockwise around the Z axis of the ecliptic (image)

Using ephemeris from the NASA/JPL Small Body Database, move Cacus to its position in orbit at JD0. (image)
Move the camera to Earth's position at JD0 and point it at the asteroid. (image)

My final rendering does not match the projection from DAMIT.  I understand that there are factors like light-time correction that I'm not including, but it looks completely different.

What is wrong with my approach to rotating the asteroid on its spin axis?

(The work-in-progress visualization is online here)


texts: What is the current accepted theory for Neptune's immense wind speed?

texts:
According to "Neptune: The Other Blue Planet in Our Solar System" (Choi, 2010), Neptune, despite its great distance from the sun, is the windiest planet in the solar system, with winds reaching 2000km/h (1500 mph) as detected by the Voyager 2 probe in 1989.

Given that Neptune is 30 AU from the Sun, what are the currently accepted theories for where the planet gets energy for these immense wind speeds?


texts: What is radio-emitting gas? (I think it refers to the Magellanic Stream, but I can't be convinced.)


texts: Does the accelerating expansion of the Universe contradict Hubble's law?

texts:
Hubble's law gives a linear relationship between the distance to a galaxy and it's recessional speed.

Observations of distant type 1a supernovae showed that their red shift (and therefore their recessional velocities) were lower than expect, implying that the rate of expansion of the Universe was lower in the past.

However, doesn't this mean that Hubble's law only applies for short distances, as at great distances a plot of recessional velocities against distance would not be linear (as I have crudely tried to show on the graph below)?


texts: Why does higher surface gravity broaden spectral lines?

texts: It's always made intuitive sense to me that a higher surface gravity leads to broader spectral lines from a star, but yesterday I realized that I don't have a physical explanation for why this is so.  Why is this so?


texts: Journal standard for plotting section of skymap?

texts:
I'm a physics graduate student hoping to plot ~10°x10° sections of the sky in right ascension and declination (eventually I will want to plot points on this section at appropriate locations given RA and Dec, preferably in Python). I've seen square plots on the order of arcseconds, where curvature has a small effect, but that won't apply to these larger plots. I'm not familiar with astronomy journals or a module which aids in this sort of plotting.

Is there a commonly accepted method for displaying intermediate-sized sections of a sky-map? How is it implemented?


texts:
This week we have read of evidence of the Super Massive Black Hole in the centre of the Galaxy 'flaring'.

My understanding is that light cannot escape from a black hole.

My question is: How does a Super-Massive Black Hole 'flare'?


texts: Why is the number of Long Gamma Ray Burst (LGRB) event occurrences greater than the number of Short Gamma Ray Burst (SGRB) event occurrences?

texts:
I collected  from NASA this histogram plotting the number of GRBs vs the duration of GRBs.

Now my question is, why is the number of Long Gamma Ray Bursts (LGRBs) events occurrences greater than the number of Short Gamma Ray Bursts (SGRBs) event occurences, since on that plot the area covered by the long duration GRBs is greater than short duration GRBs?


texts: Why is the time between equinoxes different?

texts:
The path of the Earth around the sun is described as being an ellipse, with the Sun being at one of the foci of the ellipse.
Since an ellipse, by definition, is symmetrical with respect to the foci, I would expect the time for the earth to transit any two halves of the ellipse to be the same, so long as it moves at the same speed around the ellipse, or at least the speed characteristics are symmetrical as well.
However, this does not appear to be the case according to the following rounded figures for the time in days between equinoxes and solstices:
Summer Solstice to Winter Solstice    184.3
Winter Solstice to Summer Solstice    181.0
Vernal Equinox to Autumnal Equinox    186.4
Autumnal Equinox to Vernal Equinox    178.9

These values can be found, for example, in astronomer Robert Newton's book of 1976.
Why are the values different?


texts: How could the Moon have arisen from a collision - what would make for an elliptical orbit?

texts:
One serious theory advanced for the creation of Earth's moon is that it resulted from a collision.

However matter would be thrown off in all directions and some will fall back. How could a mass of ejecta as large as the Moon all end up travelling together in a single direction and therefore a single orbit? For a spacecraft to attain orbit requires some form of propulsion. Even if the impacting object 'bounced off' I'm still not clear that it could end up where the Moon is and travelling in the right direction. This would require it to be 238,900 miles away and travelling roughly at right angles to the Earth's gravitational field.

  The giant-impact hypothesis, sometimes called the Big Splash, or the
  Theia Impact suggests that the Moon formed out of the debris left over
  from a collision between Earth and an astronomical body the size of
  Mars

  https://en.wikipedia.org/wiki/Orbit_of_the_Moon


texts: Can two detectors for gravitational waves be combined into a very long baseline interferometer?

texts: While detectors for gravitational waves are interferometers themselves, the question here is about an increase in resolution resulting from two or more detectors far away from each other.


texts:
According to Google, radial velocity is "the velocity of a star or other body along the line of sight of an observer".

The radial velocity of Sirius according to Google is    −5.50 km/s.

If the number is positive, does that mean it's moving away or towards us?


texts:
How is a Dead Comet different from the normal comet?

How are they formed?

And why is the Halloween asteroid 2015 TB145 called a dead comet?.


texts:
Astronomers recently claim that there is evidence for a 9th planet. As far as I understood it, this is mainly based on coinciding orbital parameters of a few Kuiper belt objects.

Could the 9th planet be a virtual planet? I mean this in the sense that we see the gravitational influence of a multi-body system. Could the 9th planet simply be the barycenter of the Kuiper belt and its apparent orbit some form of precession?

Apart from the question, if this was a possible or likely scenario, how could we distinguish a "real" 9th planet from a virtual one, except by the obvious answer if we can observe it directly?

Forgive me, if this is a stupid or trivial question or if this was answered before. I am not a professional astronomer, yet I have a physics/engineering background. I did a quick internet search to answer this question but I simply may have used the wrong terms or sources.


texts: How to estimate age of asteroid family Erigon

texts:
I have one problem:

  On picture below we can see Erigone family asteroids in plane semimajor axis - absolute magnitude. Based on V-type as a consequence of Yarkovsky effect, estimate age of this family bearing in mind that members of this family have spectral of C-type. For average value of change of semimajor axis which belongs to this family take $da=5 \times 10^{-4} \text{AU}$ (for object with diameter $D=1 \,\text{km}$ and time $t=1$ million years). Ignore initial size of family.

I really don't know how to solve this problem (I am studying math, not astronomy). Professor won't tell us how to do it, and I tried to find the answer in his textbook and in two more books in my language, but there is no similar problem at all.

Could you point me in the right direction?


texts: Which lens is right to see which planets with an Orion StarSeeker 150 mm?

texts:
I'm an amateur astronomy lover and I have an Orion StarSeeker IV 150 mm, together with a bunch of lenses, which are the following:

1x 23mm (32.6x) and 1x 10mm (75x) wide-field 60-degree eyepieces 1x Plössl 32mm 1x Plössl 12,5mm 1x Plössl 6mm 1x Plössl 4mm 1x Barlow lens 2x 1x green filter No.56 1x red filter No. 23A 1x yellow filter No.12 1x orange filter No.21 1x blue filter No.82A 1x polarizer filter 1x lunar filter Crystal View.

I've been able to see the moon using the Barlow Lens and a 10mm lens, but I can't see other planets, star clusters and other stellar objects. I've searched on the internet, but haven't been able to find information that is understandable for an amateur, so here I am.

Which lens is best to see which planets and star clusters?

Thanks to whoever will help me :)


texts: How to determine the orbital period of an asteroid using Kepler laws given its orbit's radius?

texts:
Given that the mean distance of an asteroid from the Sun is $450 \times 10^6$ km is it possible that the asteroid completes one rotation around the Sun in two years?

According to the third law of Kepler the farther a planet if from the Sun the longer is its orbital period. Because the distance from the Sun to earth is $149 \times 10^6$ km and it takes a year for earth to complete the rotation then for the asteroid it would take 3 years.

I tried to actually calculate how much time would take for the asteroid to complete one rotation combining Newton's law of gravitation and Kepler's third law and given $G=6.673 \times 10^{-11} Nm^2/kg^2$ and converting all distances to meters:

$$
T^2=\frac{4\pi^2r^3}{G\cdot M_{Sun}}=\frac{4\pi^2\cdot 450^3\cdot 10^{21}}{6.673\cdot 10^{-11}\cdot 1.989\cdot 10^{30}}=\frac{4\pi^2\cdot 450^3\cdot 10^2}{6.673\cdot 1.989}=2.99\cdot 450^3\cdot 10^2=27246375\cdot 10^3.
$$

Then $T=\sqrt{27246375\cdot 10^3}=165064$ seconds $=45$ hours. Is there something wrong in my calculations or I'm not using the correct formula?


texts: Rotation of far away and near by galaxies

texts: Do the far galaxies rotate the same as does the milky way and the near by galaxies?


texts: Planetary observations: what to expect from the Powerseeker 50AZ?

texts:
I bought the Powerseeker 50AZ, which has D=50mm; F=600mm F/12 which comes with 3 eyepieces: 20mm, 12.5mm and 4mm.

As far as I can tell it's not possible to colimate this telescope. As suggested here this is a very important part to see sharp images.

I have tried to observe jupiter and mars with it and jupiter I could see a dim small blurry image and mars a very tiny dot with absolutely no way of seeing details on any of those in what seemed the best focus I could get from them.
The only thing I can see details is the moon really. Also I have observed in what seemed very clear sky conditions.

So my question is:
How much can I expect to see from planets with this telescope?
With this current setup should I be seeing jupiter and mars in more details?

UPDATE:
Equipment link

https://www.amazon.com/Celestron-PowerSeeker-50-Refractor-Telescope/dp/B0000UMLYI/ref=sr_1_1?ie=UTF8&qid=1515177254&sr=8-1&keywords=powerseeker%2B50az%2Btelescope&th=1


texts: Could dark matter assist with star formation?

texts: This is related to the question: "Can dark matter decrease the Jeans length?" If dark matter assists in any way with star formation, shouldn't it be detectable by precisely measuring velocities in young open clusters? In particular, could the motions of stars in the Pleiades all be explained without the presence of dark matter?


texts: Does the radius of the Universe correspond to its total entropy?

texts:
I heard a claim that due to holographic principle, the surface area of the cosmic horizon corresponds to the universe's total entropy.

As such the initial state had zero surface area and later expanded.

Given this, I wonder whether any increase in entropy (such as producing heat by means of electric power) causes the universe to expand?


texts: How can I check if we can observe a certain star (given its equatorial coordinates) from a certain location on Earth (given its DMS coordinates)?

texts: I am given the equatorial coordinates of a star (e.g. Sirius has RA: 6h45m8.9s and Dec = -16°42'52.1"). Now my question is: can we observe this star from a specific location on Earth (e.g. from Brussels 50°51'1.62" N 4°20'55.61" E)? If yes, how can I justify this using the coordinates of the star and location on Earth?


texts: What is the status of gravitational wave detections in the CMB?

texts:
I've not been in the loop for this, but can someone tell me what the status is of primordial gravitational wave detections in the CMB?
This can essentially be detected through B-mode polarization observations, if I'm not mistaken, but those would be significantly weaker than the E-mode signal. Has the Planck team managed to make any strides in this?


texts: Approximately how many light years across (arclength) was the furthest reaches of the Hubble Ultra Deep Field?

texts: I can find the area($11^2$ arcminutes) and lengths ($200$ arcseconds per side) but if I were to consider just the furthest region (arclength) of the survey, about how many light years across would this be?


texts: How do I get flux density when I have mJy/beam , beam size and pixel size in an astronomical image

texts: I have a fits file of the galaxy m87 produced by SCUBA 2 and I am wondering how I obtain the fluxes at 450 and 850 microns. the output image gives pixel magnitudes in mJy/beam where I want to find eh flux for a certain part of the galaxy. I am as far as summit the values of all pixels within an aperture but I am not sure where to go from there to obtain the flux as there are more than one pixel within the FWHM. say I have a an aperture of radius 1.5 pixels that gives a sum of 6985.04885203 mJy/beam with a pixel size of 3.99999999 arc seconds and a beam size of 13 arc seconds how do I obtain the flux for that aperture at that wavelength?


texts: Determining wind speed distribution on Mars using dune shapes?

texts:
As a fan of exometeorology (the study of atmospheres of other planets), I scanned through a whitepaper Measuring Mars Atmospheric Winds from Orbit which says

Measurements of Mars
atmospheric winds from orbit would dramatically advance our understanding of Mars and help prepare for human exploration of the Red Planet.  Multiple instrument candidates are in development and will be ready for flight in the next decade. We urge the Decadal Survey to make these measurements a priority for 2023-2032.

This gives me the impression that our knowledge about spatial wind speed distribution on Mars are scarce. But what about indirect methods to determine prevailing wind speeds at different locations on Mars?
Some years ago I attended a lecture by Prof. Hans Herrmann showing that the shapes of Barchan sand dunes might be a good candidate for indirectly determining the prevailing winds of Mars. However, I could not find any survey study which would analyze the complete observed Mars surface for dunes and map the average wind speed on Mars. Is there such an effort already on the way? Are there any arguments against such a Mars-dune-windspeed-survey?
Further reading

Philippe Claudin, Bruno Andreotti: A scaling law for aeolian dunes on Mars, Venus, Earth, and for subaqueous ripples. cond-mat/0603656
Eric J. R. Parteli, Hans J. Herrmann : Dune formation on the present Mars arxiv:0705.0809
ESA.int: The effect of the winds of Mars
Is the wind's intensity on Mars similar to Earth?


texts:
“Who saw” the binary neutron star merger first? What was the sequence of events? (GRB/GW170817) highlights a particularly notable astronomical alert, but I am sure that there are alerts triggered by various systems sent to various other systems.
Question: Is there a standardized "Astronomical Alert" system? Or are there many decentralized and specialized systems for each of various specific scenarios, such as the one described in Does the Zwicky Transient Facility only run when triggered by a cell phone?
The question occurred to me after posting Will LSST make a significant increase in the rate of astronomical event alerts? and reading the excelent answer(s) there!


texts: What are recent research techniques to find Dark matter? As they don't interact with other matters, which properties of it can help us to detect it?


texts: Why is the Lunar Node in a daily motion?

texts:
I've read a lot about the Lunar Nodes online (on Wikipedia, astro.com and a bunch of other websites), but there is one thing I cannot figure out and the sources don't seem to clearly explain: why are the nodes moving on a daily basis? As the node is a point of intersection of Moon's orbit and the Ecliptic I understand that this point does not fall in the same place in space every time, but I would expect that once it's passed by the Moon it stays where the crossing occurred and its position gets updated on the next crossing of the Ecliptic, so in ephemeris I would expect to see the same values of the coords between the crossings, but instead I can see they're changing on the daily basis (ie. this ephemeris has 2 columns showing true and mean nodes' positions: http://www.astro.com/swisseph/ae/2000/ae_2016.pdf).
I'm sure the ephemeris cannot be wrong, so I must be missing some details. I thought that finding the Moon's nodes would be as easy as finding the dates/times/longitudes when the Moon's latitude equals 0, but apparently this way I can only find the times and static points of the crossings of the Ecliptic, not the moving nodes as they're shown in ephemeris tables.


texts: How big will Apophis appear in the sky at its closest point of approach in 2029? Will it compare  to the moon? How long will it linger as it passes?


texts: Software with simple representation of all stars

texts: Do you know any program similar to TheSky (astronomy software), which enables us to identify stars after clicking on them with a simple view (like in the figure)? Many thanks


texts: Replacing Meade ETX with computerized mount

texts:
I've been using a Meade ETX 90 for a few years now. Since it doesn't have the new computerized mount (with goto), I am looking into buying one to swap out. Where might I find this and is there a more technical term than just "computerized mount" that is used?

Note: This is an old scope that did not originally come with the ports that the autostar controller now goes into (on the newer models).

Update: I ended up getting a newer Meade ETX and plan to replace my current mount with the computerized one. Also, if you check out cloudynights.com you might be able to find one. I have seen one on there before.


texts: Calculating the Angular Momentum of a planet

texts:
I want to calculate the Angular Momentum of any planet at any point on the orbit around the Sun.
Example: I want to calculate the Angular Momentum of Pluto today, on Jan 27, 2021 00:00:00 Hrs
Where can I get the actual data to calculate this?
I will need Mass, Velocity and Radius to calculate. Do I need the θ too?
Is it readily calculated by NASA?
http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html


texts: Calculating Radial Velocity from Hipparcos

texts:
Using data from Hipparcos is it possible to calculate Radial Velocity.  If we take 19 Leonis as example, RV is -3.70.  Is it calculated from Proper Motion?  Can I ask that any formulas be done in long hand please, i.e. how you would enter it into a standard spreadsheet rather than with symbols etc.

http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=19+Leonis&submit=SIMBAD+search

Additionally

Took the advice and found CRVAD2.dat ( http://cdsarc.u-strasbg.fr/viz-bin/Cat?III/254 ).  The following line comes from CRVAD2.dat file and when I looked up HipId 40829 which is below, the RV value is -3.70 which matches the link above.

9.790554129  11.56817726   0.8   0.1   11.06   1.00   -48.63    -3.39   1.24   0.69  6.709  6.436 0.005 0.005 0.007   D   AY D A3n          A 7 VN                48029  84722  948942 4 ABA    -3.70  -9.99 B   9 1

If I look up Sirius (32349), it says the RV is -7.60 but when you look up Sirius at Simbad, it says -5.50 ( http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=Sirius )

6.752569403 -16.71314306   1.2   0.1  379.20   1.58  -552.63 -1206.40   0.50   0.50 -1.430 -1.121 0.011 0.002 0.039   U     OD A1V+A5wd     A1V+A5wd              32349  48915 1447791 4 AB     -7.60  -9.99 A   0 1

Whats the reason for the discrepancy, CRVAD2.dat is an old version like there's two Hip data files (97 and 07)?


texts: Freefall timescale for a Jeans-unstable hydrogen cloud of mass M

texts:
I'm working on a project (personal, not academic) that involves calculating the collapse timescales for protostars which will end up becoming stars of varying given masses.  I'm treating the pre-main sequence part of the star's life separately, so I'm only concerned (for now) with the initial period of rapid collapse of the protostellar dust cloud.

My initial idea was to calculate the density of a gas cloud that would have a Jeans mass equal to the mass I'm interested in, and then use that to calculate the freefall time for a cloud with that density.  I'm working with the equations found here, which I believe are correct but I'm no astrophysicist so I may not notice mistakes if the page has any.  For my purposes, treating the interstellar medium as 100% H$_2$ at a uniform temperature of 20K is a good enough approximation.

Given the above, I derived:

$$ n\approx\frac{4.2\times10^{69}\frac{kg^2}{m^3\cdot K^3}T^3}{M_J^2}\Rightarrow t_{ff}\approx\frac{84580\frac{s\cdot kg^{\frac{1}{2}}}{m^{\frac{3}{2}}}}{\sqrt{\frac{56.2\times10^{45}\frac{kg^3}{m^3\cdot K^3}}{M_J^2}}}$$

Everything is in SI units.  So, for example, say I know that I want a star which will be one solar mass when it reaches ZAMS.  I take $M_J$ to be one solar mass, and I find that I get a cloud density of about 8.48$\times$10$^{12}$ molecules per cubic meter, which I believe is quite reasonable for a molecular cloud, and a freefall time of about 23,000 years (which seems a bit short to me, but again I'm no astrophysicist).

For stars with more extreme masses, I don't have any idea if my results are reasonable.  Here're a couple of examples: if I want a star with 0.07 solar masses, I get a freefall time of only about 6000 years, from a cloud of density 1.73$\times$10$^{15}$ molecules per cubic meter.  This seems unrealistically dense for a gas cloud, but maybe you could get densities like that during fragmentation of a larger Jeans-unstable region?  For a star which will be 150 solar masses, I get a freefall time of 280,000 years (which is more like what I was expecting for one solar mass) and a cloud density of 3.77$\times$10$^8$ molecules per cubic meter.

My questions are basically: is this approach reasonable, and do these results seem at least passingly plausible?  If this approach is sound, I've already derived equations for radius, luminosity, and effective temperature of the collapsing cloud as functions of time and mass, so I'm hoping my work to this point hasn't been completely wasted, but I'd be happy to use another approximation method if someone could suggest a better one.


texts: Force to fix $\sigma_{8}$ or $A_{s}$ for Forecasts

texts:
In the context of Forecasts with Fisher's formalism, I make vary cosmological parameters to compute the elements of the Fisher matrix.

First, I generate with CAMB code a linear power spectrum. Then, from this, I am computing $\sigma_{8,\text{linear}}$.

Secondly, Before relaunching the code CAMB in non-linear regime, I apply a correction on $A_s$ (amplitude of primordial power spectrum), by a simpe relation of proportionality on the 2 $\sigma8$ (fiducial and linear), to get a new $A_{s,\text{modified}}$  that will procude the same $\sigma_{8,\text{fiducial}}$ after the non-linear regime execution of CAMB code.

For example, if $\sigma_{8,\text{linear}}$ is higher than $\sigma_{8,\text{fiducial}}$, I do the following correction before launching the non-linear CAMB regime :

$A_{s,\text{modified}} = A_{s,\text{fiducial}}\,\bigg(\dfrac{\sigma_{8,\text{fiducial}}}{\sigma_{8,\text{linear}}}\bigg)^2$

So, $A_{s,\text{modified}}$ will be smaller than $A_{s,\text{fiducial}}$.

My main issue of understanding :

1) Why have we got to do this correction ?, I mean to compute a new $A_{s,\text{modified}}$ that will give a new $\sigma_{8,\text{non_linear}}$ equal to $\sigma_{8,\text{fiducial}}$ (or maybe $\sigma8_{linear}$, I am not sure from the relation of proportionality above) ?

For the moment, I think we want to keep a fixed value for $\sigma_8$ to be consistent with observations data where $\sigma_8$ doesn't change (does $\sigma_8$ always correspond implicitly to amplitude of fluctuations at $z =0$ ?) : but I am not sure to grasp this subtility.

2) By the way, why we don't compute only once directly the non-linear regime instead of launching 2 times the code CAMB (first in linear regime and second in non-linear regime with this correction between both) ?

Thanks in advance for your help, even a succinct answer would be nice.

Regards


texts: My local night sky is so bright that I can only see the Moon, so would a low-cost telescope be any use?

texts:
Light pollution is so bad near our home that we can't see anything other the Moon in the night sky. For instance there is a street light on all night right in our lane and there are many other housing lights in the area.
Am I right to think that with a low-end telescope I won't be able to see anything else besides the Moon?


texts: What kinds of technologies are required to directly image exoplanets

texts:
I have read that immensley powerful telescopes such as the European Extremley Large Telescope will be able to directly image the atmospheres of exoplanets, and even determine their rotation rates.

I am wondering, what are the obstacles that prevent current telescopes from achieving this; how will these obstacles be overcome exactly by the new telescopes such as the EELT; and tentativley when can we expect these kinds of telescopes to come online?

Also, is it theoretically possible to build a telescope (earth or space based) that can discern fine structures such as clouds, or surface features of exoplanets (or is this in fact the level of detail that will be achieved by telescopes such as the EELT)?


texts:
I'm looking for some sort of boundary data to be able to render the milky way on a star map, as visible from Earth. Something that looks like this:

For that, I need something like a collection of RA hours and DEC degrees of the "boundary points" of what's visible from Earth (technically the Galactic Center), possibly with proper motion too. I'm not looking for precise luminosity data or anything like that, just the points of the blob on the sky that most resembles the Milky Way's shape and position from the Earth. It's important that I want to render the sky map for any given surface point on Earth, for any given time (within the last 100 years at least).

Do you know of a database like that? I've been looking on VizieR but I couldn't find what I was looking for.


texts: How to calculate the position of a meteor shower's radiant point based on its associated comet's orbit?

texts:
Below this answer to Why are Delta Aquariids “for the southern hemisphere” while the Perseids are “for the north”? I wrote the comment:

+1 To make this complete, Comet Swift-Tuttle has a nearly circular orbit of about 1 AU but an inclination of 113.45° which is just about what it would have to be (double) to push the Perseids' radiant to a declination of +58°.

Now I'm thinking that a vector that points toward's a meteor shower's radiant is just the vector sum of the Earth's orbit's velocity vector and the velocity vector associated with the orbit of the shower's associated comet at their intersection point where the shower is a maximum.
Question: If that's the case, is there a simple set of equations for the position of a meteor shower's radiant point  and a source for them that can be cited?
It seems like this would have been done quite a long time ago and should be found in some classic early work!

For additional reading and resources about the relationship between a meteor shower and it's associated comet's orbit, see:

Why do particles from a comet that result in meteor showers spread out mostly along the comet's orbit?
Have meteor showers been predicted for planets or solar-system bodies other than Earth? Have attempts been made to detect them?
How can comets have tails if there's no air resistance in space?
Why would the Perseids meteor rate fall off after maximum faster than the increase before maximum?
Meteor shower predicted to be best viewed BEFORE midnight (for reasons other than the moon)?
What makes some meteor showers continue for days, while the "Unicorn shower" can be shorter than one hour?
How significant are the recent results that Scholz’s Star has perturbed several observed hyperbolic objects?
Roughly what fraction of comet orbits that intersect Earths' orbit result in known meteor showers?
Why are these objects moving at Vastly Different Speeds along the same orbit?


texts: Is there an online planetarium where the observer is on another celestial body?

texts: I know about in-the-sky.org and its online planetarium feature, but that's from the Earth's perspective. Is there a planetarium (preferably online but if not, that's fine) where the observer is on another celestial body, like the Moon or Mars?


texts: First Telescope SkyWatcher Heritage 5" Tabletop Dobsonian Telescope

texts:
I have been star gazing for awhile with a pair of binoculars and now I am ready to step it up a bit and purchase a telescope.

I have been looking at the SkyWatcher Heritage 5" Tabletop Dobsonian Telescope.

Would this be a suitable first telescope , I hope to be able to make out the rings of Saturn using stock the Eyepiece.

Or would this be considered overkill and something like the Celestron 21035 70mm Travel Scope be more suitable.


texts: How strong is the force between the Sun and the centre of the Milky Way?

texts: I know the Sun is orbiting around the Milky Way, but how strong is the attractive force between them (e.g. what is the order of magnitude in terms of newtons)?


texts: How do I calculate Allan Variance (maybe for pulsars)?

texts:
I am studying pulsar timing this summer and have come across many Allan Deviation plots that include several popular clocks alongside pulsar measurements, like this one by N. Ashby and D.A. Howe:

I am unclear as to where to start. What equation would be appropriate? I have pulsar timing data from the NANOGrav Public Data Set, which I am able to view through Tempo2 (aka will be able to view it as soon as I figure out how Tempo2 works). If there is other software anyone would recommend for this task that would also be helpful.

I am also unsure where the data for the other clocks would be publicly available. I am hoping a scientist who has done a similar calculation can point me in the right direction.


texts: How can I improve the Field of View of my home-made telescope?

texts:
I made my telescope, with two convex lens of focal length 1000mm and eyepiece (plano-convex) with 25mm focal length. But I can see only a very small field of view, with inverted image. Do I need to increase the diameter lenses for more area capture, and can I use concave as eyepiece for upright image?
Or does some different solution exit?
I can read all information related to telescopes and binoculars, ie. how lenses work, which lenses combination are for which purposes (terrestrial and astronomical), and increase or decrease area and magnification.
Please note here I am talking about only refracting telescope, and binoculars.
note: This is a follow-up question based on comments below this answer to my previous question.


texts: Why does light from objects take time to reach us?

texts:
Why does the light of objects such as the Sun, stars etc. take time to reach us?

E.g. Why does sunlight take 8 minutes to reach us, instead of reaching our eyes as soon as it rises.


texts: Thought experiment about our location in the Universe

texts:
[I am sorry if this isn't the best place for this - it was either here or maybe on the Worldbuilding SE.]

I recently re-read one of my favourite childhood sci-fi books (Chocky by John Wyndham), and in it a young boy is asked by an alien being, where in the Universe our planet is.

It occurred to me that this is almost impossible to describe.

Nowadays, with Wikipedia and the Internet, we can get some of the way by stating that we're in a galaxy that is approximately 10 billion years old, light takes approximately 100000 years to cross it, and which is part of a local group, itself part of a cluster, and that part of a super-cluster.

Unfortunately the alien might then ask a follow-up clarification question such as what is a year?

So we go to basics - in base 60 (as opposed to binary or base 10), our basic unit of time is called a second, and there are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a day - which is the time our planet takes to rotate once completely. Our planet orbits a local star (that we call the sun) and each complete orbit takes approximately one year, and light takes around 8 minutes to get from our local star to our planet.

Unfortunately our alien might then ask, but how long is a second? - so we then get into atomic clocks, caesium 133 and phases of radiation, followed by the next inevitable question: what is caesium. So we have to get down to the Periodic Table of the Elements and decribe an atom, atomic weight, and how to define caesium in a way that a far off advance civilization could relate to - not to mention perhaps a tangent about Prime Numbers.

So if our alien was lucky enough to be in contact with someone like Professor Brian Cox, or a physyicist with astronomical leanings, how would such a description and conversation go?


texts: What is the current accepted theory for the fate of hot Jupiters?

texts:
It is well established that one main feature of many hot jupiters is their close proximity to their parent star, usually the equivalent of being within the orbit of Mercury.  So, these planets are gas giants and are very hot (hence their category).

However, a few discoveries have lead to questioning about what is the fate of these planets.

example 1: HD 209458b  a.k.a. "Osiris"

According to the NASA page "Dying Planet Leaks Carbon-Oxygen", Osiris is doing more than 'evaporating', it is leaking carbon, oxygen alongside hydrogen in an envelope behind the planet that has been detected from Earth.  The significance of carbon and oxygen is stated from the article:

  Although carbon and oxygen have been observed on Jupiter and Saturn, it is always in combined form as methane and water deep in the atmosphere. In HD 209458b the chemicals are broken down into the basic elements. But on Jupiter or Saturn, even as elements, they would still remain invisible low in the atmosphere. The fact that they are visible in the upper atmosphere of HD 209458b confirms that atmospheric 'blow off' is occurring.

It is stated in the article that Osiris is likely to become a hypothesised class of exoplanet known as a Chthonian, which is defined in "Evaporation rate of hot Jupiters and formation of Chthonian planets" (Hebard et al. 2003) as

  new class of planets made of the residual central core of former hot
  Jupiters

These would be similar in size to Earth, but considerably denser.

example 2: CoRoT-7b

According to the NASA article "Most Earthlike Exoplanet Started out as Gas Giant", CoRoT-7b is an Earth-sized planet where a hot Jupiter usually is found, they describe it as

  is almost 60 times closer to its star than Earth, so the star appears almost 360 times larger than the sun does in our sky," Jackson said. As a consequence, the planet's surface experiences extreme heating that may reach 3,600 degrees Fahrenheit on the daylight side. CoRoT-7b's size (70 percent larger than Earth) and mass (4.8 times Earth's) indicate that the world is probably made of rocky materials.

The high day time temperature means that the star-facing side of the planet is likely to be molten, any tenuous atmosphere is also blasted away.  Scientists estimate that many Earth-masses may have been boiled off.  It also seems that the decreasing mass is causing the planet to be drawn closer to the star  -cause more material to be boiled off, hence the mass to decrease.

To summarise on of the scientists in the article:

  You could say that, one way or the other, this planet is disappearing before our eyes,"

The question

As these are only 2 examples of a possible process, the question is, what is the current accepted theory as to the fate of hot Jupiter exoplanets?

Could this also be the reason a hot Jupiter does not exist in our solar system?


texts: Do all planets have a van Allen radiation belt?

texts: Earth has the van Allen radiation belts. Which other planets have one? Can it be determined if distant planets in other solar systems have one?


texts: Orbiting supermassive black hole or galactic center of mass?

texts:
One of the ways they measure the (supposed?) supermassive black hole at the galactic center of the milky way is to measure those tens of stars right at the galactic center that are orbiting what appears to be nothing, but what is said to be a supermassive black hole.

My question is why there is a distinction between all other stars in the galaxy orbiting (for instance, our own sun) and those few right near the galactic center orbiting.  My point is, if you ask what our solar system orbits, an astronomer wouldn't say "the supermassive black hole at the center," he would say "the galactic center of mass."  My assumption is that you couldn't even calculate the mass of the supermassive black hole by knowing our sun's mass and orbit - people would probably say we are too far out, or we are orbiting the galactic center of mass rather than the black hole, yadda yadda.

Questions:

1) Why the distinction?  How do we know that the stars right at the galactic center aren't also orbiting the galactic center of mass vs. the supermassive black hole?

2) What would happen if you magically removed the supermassive black hole in an instant?  Would the stars right at the center continue to orbit the center?  If so, how is that different than what they were originally doing?

Note that I know that center of mass of the galaxy and the supermassive black hole are two different things.  I also know that the center of mass of the galaxy is either superimposed on the supermassive black hole, or quite close to it.  I just want to know how or why some objects feel the center of mass of the galaxy and respond to it, while others feel the pull of the supermassive black hole and respond to it.  How far away do you have to be to be considered "orbiting the center of mass" rather than "orbiting the supermassive black hole?"


texts: Why does this twilight sky flat field have a grid of dark pixels?

texts:
The image below is a small region of a twilight flat field. A grid of darker pixels with 728x36 cell dimensions can be seen.

The camera used was the SBIG STL-6303 according to the FIT headers, which uses a Kodak KAF-6303E CCD according to the online data sheet.

The CCD dimensions, these cell dimensions, and all the binning options are all perfectly divisible, so I'm assuming this artifact is due to the structure of the CCD.

I plotted the mean pixel value of each row to make sure this wasn't an artifact from the image viewing software. The plot clearly shows a drop every 36 pixels.

So, I'm hoping for some confirmation: is this decreased sensitivity due to the structure of the CCD?

If so, what about the CCD structure actually causes this?


texts:
In a discussion below the Space Exploration question Are there any to-scale diagrams of the TRAPPIST-1 system I was invited to ask a question like this, so I have.

Question: Are the planets in the TRAPPIST-1 system so close that inhabitants on one planet could see surface details on the other planets? How big would each planet appear from each of the other planets during conjunction or opposition? Bigger than Earth's moon?


texts: How does a spinning black hole affect space-time compared to a non-spinning black hole?

texts: I often see questions on this site that only apply to non-spinning black holes and often wonder how the answer would be different for a spinning black hole.  As the title says, what are the major differences between the behavior of spinning and non-spinning black holes, especially when it comes to the effect on space-time?


texts: Do the magnetic fields of stars and/or other planets reverse?

texts: I've heard of the magnetic field of the earth reversing poles over the course of thousands of years but do stars and other planets (specifically gas giants) also experience this? If so how does it differ and if not why?


texts: Relationship between metallicity and color? Should Pop. I stars be blue?

texts:
I have found in numerous places such as this website: http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structure/metals.html or in "Introduction to Stellar Astrophysics" by B. Carroll, that state:

  Population I:
  metal rich [Fe/H] > -1
  disk stars
  open clusters
  metal rich makes stars redder

  Population II
  metal poor [Fe/H] < -1
  halo stars
  globular clusters
  metal poor makes stars bluer

And I also found (from: http://hyperphysics.phy-astr.gsu.edu/hbase/Starlog/pop12.html):

  Population I stars include the sun and tend to be luminous, hot and young, concentrated in the disks of spiral galaxies. They are particularly found in the spiral arms. With the model of heavy element formation in supernovae, this suggests that the gas from which they formed had been seeded with the heavy elements formed from previous giant stars. About 2% of the total belong to Population I.

  Population II stars tend to be found in globular clusters and the nucleus of a galaxy. They tend to be older, less luminous and cooler than Population I stars. They have fewer heavy elements, either by being older or being in regions where no heavy-element producing predecessors would be found. Astronomers often describe this condition by saying that they are "metal poor", and the "metallicity" is used as an indication of age.

If Population I stars are metal rich and therefore appear red, how can they be luminous, hot, and young, and concentrated in disk (the region of star formation)? Shouldn't they appear blue? How does color relate to metallicity?

Hot stars should be blue, shouldn't they? As stated here for example:

  A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K. (http://www.webexhibits.org/causesofcolor/18B.html)

Also, this question is a little similar but not quite: Metal rich or metal poor?

There is a graph: http://science.psu.edu/alert/images/SDSSmetals.jpg/image_view_fullscreen

I'm not sure if the colors there correspond to the colors of the stars, but they used blue to refer to old stars. Why?


texts:
According to cosmological constant, dark energy should have a constant density but since the universe is expanding so is the volume of the universe and so should the mass of dark energy.

But how does dark energy gain mass or energy?


texts: How to find a deep sky object 1° NE from a given star

texts:
I am an amateur so sorry for any misunderstanding.

I am trying to find a deep-sky object located 1° north east from the δ Cas star. I am using the Stellarium program to do this.

I have toggled on the equatorial grid and Cardinal points are displayed as well, however, all of the cardinal points appear to be on circumference and I cannot really tell which direction is the wanted object at.

What am I doing wrong?


texts: Would the WISE Survey pick up a large planetary collisions like Theia-Earth? And from how far?

texts:
WISE or Wide-Field Infrared Survey Explorer

Large body collisions are obviously, very very rare, locally speaking, and with stars, often spiraling inwards not direct collisions.  I know that stellar collisions or gradual inward spirals have been observed, but those are much larger and easier to see from far away.

If an impact, like Theia-Earth happened in another solar-system, creating perhaps a synestia or spiraling rock-vapor world, approaching some star's surface temperature, can we say about how close that would need to be to be observed with today's instruments?

Would the outer-most debris cool too quickly and block a lot of the visibility?

My guess is, events like this would be too infrequent to happen within our view, so we shouldn't expect to observe it until we have much better telescopes, but other events weren't expected to be observed, like the gravitational waves from two colliding black holes and that was detected.

If it was close enough the heat from collision and rate of cooling should create a unique signature I would think, so if a planet-planet collision did happen in our general vicinity, we'd recognize it for what it was.


texts: Sunrise - from earth - sun position or when sun rays reaches earth?

texts:
Time for the sun rays to reach earth is approximately 8 mins.

If time of the sun rise is 7:30 - which time is this? Time when sun rays reaches earth or time when earth and sun comes into correct position where sun is visible again after night?

If it is time when sun rays reaching earth - that means sun is already 8 mins away from that first position.

If it is second option - it means it's still 8 mins to wait until we actually see 'sun rising' - sun rays reaching earth.

So which time sun rise really is?


texts:
Pretty much everything in our solar system has collisions with smaller objects that lead to an increase in the mass of the larger.  With Pluto being one of the largest things in the area of which it resides, and being surrounded by small stuff. Assuming that it does not hit Neptune, which seems unlikely

How long will it take Pluto to grow to planet size?

Clarify or what ever the name of the largest object in the Kuiper belt turns out to be, at the time it reaches the IAU definition of planet


texts:
The asteroid 2019 OK recently passed at 77.000km of the Earth, with an estimated size of 50-130m.

But if this asteroid had crashed onto the Earth, what would have been its kinetic energy on impact? And how would its mass have been affected by the atmosphere?


texts: Does General Relativity predict anything about the creating of a singularity, once enough matter collapses inward to form an event horizon?

texts:
Ignoring quantum gravity, which we don't know much about, current theory (GR) states that an event horizon forms when there is enough matter/energy density to create it, and logically, a singularity I guess forms a tiny fraction of a second later.
I know from other reading that in GR:

the event horizon forms when enough matter/energy density occurs in a region of space that all futures point inward (this is the point we can say a BH comes into existence)
that infalling matter at a black hole reaches the singularity after finite time as it perceives time, but can't see it in advance, as its never in their past light cone.  I'm not sure what the terminology/adjective is, for their time in this context, but finite time seems an intuitive term
that the notion of time and really any spacetime coordinates is complex and need choosing with care,and an easy source of confusion
that the apparent slowing/dimming/redshifting of an infalling object is effectively as perceived by outside observers, rather than the "experience" of the infalling matter, and can probably be ignored in this issue (mentioned for clarity, rather than because relevant per se)
that the singularity isn't as much a "point", as a gap or absent part of spacetime geometry.

Logically, imagining a spherical collapse, matter density and EH are achieved before the originating material reaches the "centre", so the EH presumably predates the creation of the singularity by some tiny amount of time.
So I'm curious about what GR has to say, about after an event horizon comes into existence as matter collapses inward (say), and up to/including the singularity coming into existence (if other effects don't prevent it).
Beyond the rather trite "it happens", that is.


texts:
I saw this blazing thing in the sky from Chennai, India at around 21:00 IST. Is this by any chance the Florence asteroid ? If not, what is this ? I'm pretty sure that this cannot be a firecracker because it was huge with large flames and it actually lasted in sky for at least 30 seconds till it reached heights where commercial aircrafts fly.

NOTE: This object was moving from the ground and upwards towards the atmosphere.


texts: How to tell a pulsar is rotation-powered or accretion-powered?

texts:
We need to calculate P/Pdot?

A pulsar may switch between theses two states.

For a rotation powered pulsar, it can pulse and, the pulsed Lx can be much larger than the luminosity given by its rotation?

If Lx is very small, smaller than the luminosity given by its rotation, how to tell it is rotation-powered or accretion-powered? There are not necessarily X-ray pulsations

You can see I am asking a very tricky scenario.


texts:
I'm reading up on radio astronomy, and I came across this paper from 1964. At the bottom of page 193, the author uses a unit that I've not seen before in discussing radio power emission from stars:

  Now the outbursts on the Sun give an intensity on Earth of $10^{19}$ to $10^{20}$ $wm^{-2}(c/s)^{-1}$

I'm guessing it's "Watts per square meter per [something] per second", but I'm not sure what the [something] is.

A similar unit appears in this paper on the first line on page 364:

  The comparison band in the radiometer, being separated approximately 3.25 Mc from the signal band, never encounters the hydrogen range of frequencies.

Again, this looks to me like mega[something]. Can anyone shed some light on this?

On page 362 of the second paper, the unit appears as $(Watts/M^2
)/(C/S)$ as a unit of flux. There, the $C$ looks like coulombs, but that makes the $3.25 Mc$ in the second quote seem weird.


texts: Simulating oceans and ice sheets on other planets or moons?

texts:
From my friends in geosciences I learned a lot about General Circulation Models (GCMs) (for Earth's atmosphere and oceans). I also learned about ice sheet models for floating ice (like sea ice or glacier tongues), ice on bedrock, as well as (submarine) permafrost modelling. Then, there are also GCMs amended with all these ice models.
I am aware that for model calibration purposes, one sometimes runs the latter GCMs on "snowball Earth" with all oceans being initially frozen. But:
Have GCMs have been applied to other planets or moons with liquid on or underneath its surface yet?
I consider that rather probable:

I did find the topography of venus, but did anybody run a GCM on that topography with a carbondioxid ocean?
For Enceladus, I found some publications about the topography but not the dataset itself.

Thinking about adjusting e.g. the MIT GCM to a new planet or moon, the next  challenge would be to adjust the parametrization to non-water ice or to completely different fluent, I guess. Are there other things I am overlooking?
PS: I am asking in this community as I am hoping that someone might help me out with a general recipe what to take into account when transfering a GCM from Earth to another planet or moon.
References

Is the water underneath Europa's ice cap potable? talks about the salinity of Jupiter's moon Europa.
How would water-ammonia oceans behave?


texts: Do extraterrestrial landscapes or planets count as nature?

texts:
If I am talking about the surface of Mars, is this still nature, comparable to uninhabited places on earth, or does nature exclusively describe not man-made things on earth.

I am asking this question in Astronomy, because I would like to know what astronomers or other people dealing with extraterrestrial science on a regular basis think about it.


texts: Is there a fundamental difference between a small galaxy, and a large star cluster?

texts: For instance, If a large globular cluster were somehow ejected from its galaxy, and was observed by astronomers, would they call it a galaxy, or would there be characteristics of the star cluster that make it obviously not a galaxy by definition? And why are the Magellanic Clouds dwarf galaxies, and not large star clusters? Could we remove stars until the Magellanic clouds became star clusters, or is there something intrinsically galaxy like about these objects?


texts: The data from Kepler has taught us that there is a much larger number of "Neptune-class" planets out there than we previously thought. I wonder, however, if this "dominance" of Neptunes is because of the wide range of radii we use to define this class. If I'm not mistaken, we classify a planet as "Earth-like" when it is in the range 0.75 to 1.25 R(Earth), "Super-earth" from about 1.25 to 2.0 R(Earth) and "Neptune class" from 2.0 to 6.0 R(Earth) (a much larger range which would stand out even more if we knew masses). Are we maintaining the nomenclature to classify planets of different characteristics or are we just "solar system biased"?


texts: What effect does stellar granulation to have on a chemical analysis of a star's spectrum?

texts:
The spectrum from stellar granules will obviously be for hotter gas, while the spectrum from the lanes between them will be for cooler gas.

Does this "average out" so that an average temperature distribution can be used for a metallicity analysis?
Will the spectroscopic effects be the same for all lines?


texts: Brightest most colorful star in eastern morning sky not near any constellations; rises about 4 or 5 am?

texts:
I have been believing this is Saturn, but perhaps Antares instead? It is so colorful it shines blue, red, green and white.

It is largest in sky in the East before dawn rises about 4 or 5 AM, seen from the San Joaquin valley, central California.


texts: I am looking for the SKYMAP Api (To create nightsky/star maps)

texts:
The ones used by underluckystars.com and thenightsky.com

I have found several but I am having difficulty finding a working updated version, has anyone come across this API, or know where I can find it?


texts: Paper recommendations for career guidance

texts: I'm an undergraduate student in astronomy and I'm one year from graduating, but I feel I still can't choose what career path I want to pursue. To date I've only had two courses of proper astrophysics and I liked them both. I have a notion of cosmology (CMB, B modes) and basic stellar astrophysics but I would like to explore more fields of research. Maybe this is too much to ask but it would really be of help a list of papers that describe, in general of course, different fields. It's something like this I want to read https://arxiv.org/abs/hep-ph/9512285. If you know more papers like this please share them, they could be about exoplanets, star populations, mergers, AGNs, astrochemistry, weird stars, anything. Thank you in advance.


texts: When was Mars 686 , Venus 224 and Mercury 87 days sidereal period was discovered , was it discovered by the ancients or after Copernicus


texts: Do we get any benefit from being in a galaxy?

texts:
If our solar system were to somehow be created outside of a galaxy as a single star in the vast nothingness between galaxies, would life on Earth change any?

Does being a member of a galaxy actually provide some kind of benefit to our solar system or the creation/sustaining of life?


texts: Is it possible to image a meteor so the rock is visible?

texts:
I was wondering what one would see if they were to look through a telescope tracking a meteor as it shot through the sky. Technology aside, would one be able to see the object tumbling or would it always be obscured by the light/dust/plasma?

Basically would there be any advantage to getting a high shutter-speed video with high magnification tracking the leading edge of a meteor or would it not offer any greater detail than just a bright streak in the sky?


texts: Could Ganymede's aurora be seen with the naked eye?

texts:
If one was orbiting in the Jovian system, as depicted in this artist conception,

could the aurora be seen unassisted?

Hubblesite-Illustration Credit: NASA, ESA, and G. Bacon (STScI)


texts:
It is believed that there is a black hole at the centre of the Milky Way galaxy. And the sun revolves around the centre of the milky way.
So does it mean our sun is revolving around the centre of the galaxy due to this black hole?


texts: Gravitational waves: how can scientists derrive the sizes and spin and distance of two merging black holes?

texts: After recording the ripples of  passing gravity waves I would imagine that some high-power math would be required to derive information about the sizes, spin, and distance of the merging black holes. Is it possible to get some insight into how they figured out one was 31 solar masses and the other was 20? How do you "see" spin from a wave?


texts: In this diagram about the methane concentration in the martian atmosphere, there are data points labeled "Mars year 32", "Mars year 33", and "Mars year 34". How are Mars years counted? Is there a "Mars year 0"? What is the event that defines the start of the time scale?


texts: photon gas pressure and Independence on refraction index

texts:
Brief context: when studying Stellar Structure one of the main goals, aside of determining equations that describe quantities of interest of the system, is to also determine equations of state that allows us to approximate solutions for the coupled differential equations system we find.

An important step is to find equations of state, e.g., an expression for the Pressure $P$ of the system. Following common derivations on this subject (e.g., see Rose, Advanced Stellar Astrophysics or Clayton, Principles of Stellar Evolution and Nucleosynthesis) an integral for $P$ over a cone of particles with momentum $p$ would be
$$P =\int\limits_0^{\pi/2}\int\limits_0^\infty 2 \, v_p \,\cos^2\theta \, n(\theta,p)\,d\theta\,dp$$
where $n(p$) is the gas' distribution w.r.t $p$. An example could be an ideal monoatomic gas, in the non degenerate, non relativistic case. In particular:
$$n(p) \, dp = 4\pi p^2 n \frac{\exp\left(-\frac{p^2}{2mkT}\right)}{(2\pi m kT)^{3/2}}\,dp$$
and $p = m \, v_p$ so
$$P = n k T$$

The problem: let us find then this same $P$ for a Photon Gas (radiation pressure) considering that now $n(p) \,dp$ is given by a Bose-Einstein Distribution. In particular $$n(p)\,dp = \frac{8\pi \, p^2\,dp}{h^3}\frac{1}{e^{pc/kT}-1}$$ where $p = h\nu/c$ and $v_p \overset{!}{=} c$.

The procedure is very straightforward:

$n(p)\,dp$ can be written as $$n(\nu)\,d\nu = \frac{8\pi}{c^3}\frac{\nu^2 d\nu}{e^{h\nu/kT}-1}$$ so the integral becomes $$P = \frac{8\pi h}{3c^3}\left(\frac{kT}{h}\right)^4 \int\limits_0^\infty \frac{x^3\,dx}{e^x-1}$$ where $$x \equiv \frac{h\nu}{kT}$$. Here the $\int$ (e.g., related to this question) can be either solved by using Polylogarithms,  Riemann zeta and Gamma functions, complex analysis, etc.

It's easy to check that $$P = \frac{8\pi k^4}{3c^3h^3} T^4 \zeta(4)\Gamma(4)$$ then $$P = \frac{8\pi^5 k^4}{45c^3h^3}T^4 = \frac{4\sigma}{3c} T^4$$ which matches evenly.

The question: if we're dealing with Stellar structure, potentially dense environments (the interior of a star!) then EM radiation is not necessarily propagating in vacuum (in fact, I guess it's not the case!). Then would be $v_p \overset{!}{=} n \, c  \leq c$ but this is not usually considered on calculations. Why is this? Do you know any resource where anybody have examined this and see if star interiors are affected? (it might be negligible)

Thanks in advance,


texts: What is the science behind the polar light shapes

texts:
Is there any scientific explanation that causes polar lights (aurora borealis/australis) to have their unique shapes?

Specifically:

Rayed Arcs
Dual Bands
Corona

(Images on NASA Pdf)

This site (by NASA) proposes:

  The different shapes of auroras are a mystery that scientists are still trying to
  unravel. The shape seems to depend on where in the magnetosphere the electrons
  originate, what causes them to gain their energy, and why they dive into the
  atmosphere.

(Subsection: Why do auroras come in different colors and shapes?)

I am specifically asking if there are any scientific observations (formulas or even simulations) that could describe such patterns (or even similar observations from fluid or gas dynamics). Furthermore, if it forms a rippled ring for instance, there should be a wave-like function, right?

Afterall, they could also be in a straight line..

Aurora - NASA Pdf


texts: I watched a documentary where I heard some very strange theories that are difficult for me to comprehend. It is even difficult for me to explain what I heard. "Objects that fall into a black hole leave a copy of themselves outside the black hole." "We are not living in reality but in a hologram ." Does any of this make any sense to anyone ? O.K. so I went to check the title of this documentary because how can anyone explain it without knowing what I saw. "Fingerprints of God -the Dark Universe" this is the title. I do admit that I did sleep through a good part of it because it was almost 2:00 a.m. Also if anyone wants to watch it then look for it on youtube because that is where I saw it.


texts: Why do our clocks not go out of sync with the day over a year?

texts:
Our clocks use 24 hours for a day, our planet takes 24 hours to rotate completely and our planet revolves around the sun in 365 days.
Imagine at the start of the year, the side of Earth facing Sol is 'Side A' and the dark side is 'Side B'. The side of the sun that Earth sees is 'Side A' and the opposite side is 'Side B'.
After half a year, wouldn't 'Side B' of Earth be facing 'Side B' of Sol, and thus the day/night cycle would be out of sync by 12 hours?
I've been trying to get my head around this problem for ages and I can't seem to figure it out. It seems like the angle that the sun hits the Earth must change by about a degree per day until it's gone around 360 degrees. Would that not mean the clocks offset by 1/365th of 24 hours every day over a year?


texts: Should the sun appear very low in the horizon if its path is always between the two tropics?

texts:
If the sun never rises above the tropic of cancer and never goes bellow (southern) of the tropic of capricorn then it seems that it should appear to the observer lower and lower in the horizon for as we move north. But from what I have seen this is not what I have observed traveling in northern and southern countries.
Am I not remembering right or is my expectation wrong?


texts: When we say a galaxy is 200 million light years away, does this account for the expansion of space in the time it took it's light to reach us?

texts: When we see a distant galaxy, the light we are watching started it's journey millions of years ago. In all that time, space has been expanding, so if the initial distance was A, right now the distance would be A+B and, surely, light would take more time to travel A+B than it originally did A.


texts: Are astronomers researching or trying to find signs of worm holes?

texts:
In many movies and in the popular culture wormholes are often referenced, as well as "faster than light travel", which seems almost to be the same thing, is possible.
Are astronomers seriously considering and researching this phenomenon? What could be the signature of wormholes? Would next generations telescopes as ELT able to detect such signatures?


texts: Can exoplanets be found using neutrino detectors?

texts:
I read that KamLAND can detect geoneutrinos produced by thorium and uranium decay in Earth's crust.
Could a larger detector detect neutrinos from other planets in the solar system or perhaps even exoplanets?


texts: Advice on a thread size and fitting for a Celestron binocular tripod mount

texts:
I have an older pair (8 years old) of Celestron 10X50 binoculars with what looks like a 0.25" screw hole. The closest product on the Celestron website now is this.

I have taken to looking at the moon and planets lately, with the binoculars handheld. I have a photo tripod. I got an Opticron binocular mount (this one) which fits my quick release plate perfectly. It also claims a 0.25" screw.

The problem is the the screw goes in to the hole (behind the little cap with 10X50 on it) and sticks. It goes in quite easily for 2 threads and stops. I've cleaned both, and used a tiny amount of lubricant on both, but with no joy.. Once in the mount does just about hold the binoculars, although they slew readily around the axis of the screw.

Any suggestions?


texts: Stabilizers/dampers for refractor telescope

texts: I have a basic refractor telescope. It's pretty light and so it catches vibrations and they never really settle down. Can I attach some dampers on it to cushion these?


texts: Lightning in Jupiter's "string of pearls?"

texts:
Is there any reason there couldn't be lightning in the storms that make up Jupiter's "string of pearls?"
Addendum:I'm writing a science fiction novel, with a ship passing by Jupiter and lightning is seen in one of those storms. I don't want to be in the position where someone says, "No way, there wouldn't be lightning in one of those storms."


texts: Eta Cassiopeiae star type; G0V? F9V? Both?

texts:
For a story I am looking into the star system of Eta Cassiopeiae, which is a binary with a main sequence star called Achird (very similar to the sun) and a K7V star.
The issue I am running into is the discrepancies of what the star type is; some say it is a G0V and some say it is a F9V.
I've looked at a dozen sources so I won't link anything, but I am wondering if anyone might know the most reliable and or most current source to go off of.


texts: Could all spiral-galaxies with black holes at their centers be connected via the BH themselves or Einstein-Rosen-Bridges?

texts:
If it's true that almost every spiral-galaxy has a black hole at its center,
then I have this question.
Could all these galaxies be connected via their black holes, or their Einstein-Rosen-Bridges?


texts:
If the Earth around the Sun follows a practically circular orbit of radius equal to one astronomical unit, determine its orbital speed in astronomical units per year.
If the Earth around the Sun follows a practically circular orbit of radius equal to one astronomical unit, determine its orbital speed in astronomical units per year.
A comet orbits the Sun in the same plane as Earth's orbit. The comet's closest approach to the center of the Sun is 0.5 u.a. and it has, at that point, an orbital velocity of 4pi u.a / year. What is the orientation of the comet at the moment it cuts the Earth's orbit


texts:
Is it ever possible to see Earth's shadow on other planets?
How would the shadow present, and what would be the magnification, equipment, or other conditions necessary to see it?
Is there any way of using such a phenomenon to help prove that the Earth is a spheroid?


texts: How to correctly create planetary orbits from kepler elements

texts:
I would like to build a gravity model (with 3D graphics) that would build the orbit of the planets based on orbital elements input and the gravity calculation for each element.

My initial code is live here (Click the 0.005 to turn it into 0.05 and then click on restart time to run the orbits).

The example above has all planets on Solar System (up to Neptune), the Sun and the asteroid Apophis. For clarity, the Sun and planets other than earth have been hidden, but are still considered for gravity calculations.

However, the orbits do not match the correct estimate intersection date. In this article, it says that Earth-Apophis close-encounters will happen on 13 April (meaning that the earth should be in the orbit's intersection point every 13 April, or very close to it).

The data from the program is taken from Nasa JPL.

Apophis Data
Planets Data
The sun is positioned at the origin

Start date is 2454441.5, as given in JPL Apophis data. Planets orbital elements have been corrected to this date, with the calculations in the above PDF.

Why the orbits intersection date is wrong? Should I be doing something else? Am I doing something wrong?

The above Apophis link also cites a "Standard Dynamical Model". If anyone knows how to correctly create one, I believe this would answer my question as well.

TLDR: How can I correctly create planetary orbits from kepler elements?

Thank you in advance if anyone can help me!


texts: How strong are the Lagrange Points of Earth?

texts:
How strongly are objects bound to the Lagrange points of Earth that they inhabit in them in the same way Jupiter's Trojan meteors are entrapped in its Lagrange points?

In the co-moving frame with the two main objects, how deep is the potential energy well of L4/L5?


texts:
I was looking through sky yesterday and saw something red in sky (naked eye ) like nebula or something then tried using stellarium and mobile observatory apps to find out what it was, But couldnt find anything like this in them. I also tried to capture it on my phone but nothing visible except brightest stars.

I just know it was in cygnus - vulpecula - sagitta or somewhere near it.

As i wear glasses and dont have access to dark sky, it was my first naked eye view of some sort of deep sky object. Ill become so happy if i find what that red object was.


texts: Do comets have a limited number of orbits?


texts: Can Summer Triangle asterism be used for navigation?

texts:
I am wondering, whether is it possible to use vega-altair-deneb asterism for navigational purpose? Or if there is any geometric relationship of it with Polaris?

Any idea.
Thanks


texts: Where do new stars get their hydrogen from?

texts:
When stars run out of hydrogen, they explode (though they also use heavier elements for some time) and form nebulae. In the nebulae, new stars are born which use hydrogen as their fuel.

So, my question is from where do these new stars get hydrogen for their fuel?


texts: Why isn't IMBH candidate GW190521 very "chirpy"? Which types of events will produce the most chirp-like behavior?

texts:
The first gravitational wave observations were noted for their "chirp-like" behavior; the objects would speed up as they got closer, radiating more strongly and therefore loosing energy faster. An example of a "chirp" is shown below.
The new PRL GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙ seems to show the first observation of the formation of an intermediate mass black hole (IMBH). The image below doesn't seem to exhibit much chirpiness compared to lower-mass mergers. For example from  P. B. Abbott et al. (2016)

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz...

Question: Why do some gravitational wave events chirp more than others? Which ones produce the best, longest, and most obvious chirp in frequency?

above: not very chirpy GW190521 from Abbott et al. (linked above)
below: "chirpy behavior" from this answer and P. B. Abbott et al. PRL 116, 061102 (2016) Observation of Gravitational Waves from a Binary Black Hole Merger.

See also this neutron star merger


texts: Why doesn't a singularity spin infinitely fast?

texts: A question for a random reader passing through: because of the nature of things spinning faster when it gets smaller, wouldn't a singularity spin infinitely fast as it's infinitely small, if it has a spinn from the beginning that is? Btw I know what a "ringularity" but does it automaticly become one when rotational velocity is added? Is it just "fake news"?


texts: Historical estimates of the density parameter

texts:
From the reference in this answer I learned that our current estimate for the density parameter (i.e., the density of the universe divided by the critical density, which determines the shape of the universe) is 0.9993 +/- 0.0019. But I’m also interested in the history: how our estimates of this parameter have changed since the first attempts at measuring it were made.
I have tried to find this information myself, and once again I have failed. Can anyone please help me?
I don’t need a complete year-by-year account – a few historical highlights will do. (But I do need sources.)
I have also asked this question on Physics StackExchange, but there have been no responses. I suppose this is a better forum to ask.


texts: Is it possible to have a part of space devoid of matter?

texts:
I have recently read this article and I am finding it a little hard to believe that there would be a place that is without matter. So, if there is a galaxy at the other side of the void, we would be unable to detect it. I am still very much a rookie at this, so please correct me if I'm wrong.

So could this article be telling the truth?
If so, what is the science/reasoning behind this?
What if say, a black hole were to appear in such a void, what would happen?


texts: Is it ever possible that the graviation from the mass of kinetic rotational energy will overcome centrifugal force?

texts:
This is, I think, somewhat intuitively obvious, but this comment got me thinking about this:

what is gravitational force?

"Rotation speed can create centrifugal force opposing gravity and making things lighter," Rotation also stores energy, and energy is mass, per $E=mc^2$. A body spinning sufficiently fast will exert higher gravity, e.g. a slowly-spinning neutron star will have a weaker gravitational pull than equivalent neutron star that spins very fast."

Intuitively, my answer is "no way", added mass by rotational velocity I would think, could never be extensive enough to counteract the "flicking off" or centrifugal force of very fast rotation.  Even with a Neutron Star I would think it's impossible, but I'm not 100% sure, so I thought I'd ask.

If we ignore relativity from motion but not $E=mc^2$, as that's the crux of the question

Centrifugal force = $m v^2/r$

Kinetic energy of rotation = $1/2 I w^2 = 1/5 m v^2$

Mass equivalent of kinetic energy = $1/5 m v^2/c^2$

gravitational force $g=\frac{Gm}{r^2}$

so if we apply the gravitational force of kinetic energy

$g = G(1/5 m * v^2/c^2) / r^2$, or, simplified, $G*m*v^2 / r^2 c^2$

and we compare the two equations
Centrifugal  $m v^2/r$
additional gravitational   $G*m*v^2 / r^2 c^2$

we can remove $m*v^2$ from both on the top and 1 $r$ from the bottom

additional gravitation ratio to centrifugal force = $G/r * c^2$

$G$ and $c$ are numerical.    $G$ is very small, $c$ is very big and the ratio grows smaller as the radius grows larger.

gravitational constant:   $6.67408 × 10^{-11} m^3 kg^{-1} s^{-2}
c = 2.998 x 10^8 m s-2

the ratio, unless my math is broken, centrifugal force to additional gravity from added mass by kinetic energy of motion = $1/r * 1.35 * 10^{27}$, so you'd need a hugely small r, almost a plank length or a singularity where the added gravity from kinetic energy of rotation would overcome the "flicking off the surface" or centrifugal force.

When I work out the units, I get meters per kilogram, which I don't think is right.   The units should cancel out with a ratio of two forces in opposite directions, so I think I made an error, but I don't see where I made it.

My question is two fold.    1) is my math broken? and if so, where?   and 2) is the added mass from kinetic rotational energy ever relevant, say in a very rapidly rotating Neutron star?   Could it ever assist the Neutron star in collapse or add gravity?, I can see how it could add to flattening, as rotation flattens objects naturally and perhaps, add a speck of gravity on the polls where centrifugal force is zero, but logically, I think, rotational energy would end up spinning any non black hole object apart, long before the added energy of rotation had enough mass to make a measurable difference on gravity.    Is my sense right or is there a situation where added mass from energy of rotation could overcome the centrifugal force?


texts: Do we observe Mars Eclipse or Jupiter Eclipse?

texts: We frequently observe lunar eclipse throughout the year. Have we ever observed Mars Eclipse or Jupiter Eclipse or Saturn eclipse?


texts: How are all the intermediate images between the "lensed" and "unlensed" endpoints of this video generated?

texts:
Phys.org's ALMA sees most distant Milky Way look-alike describes the numerical image reconstruction of a strongly and very nicely lensed z = 4.2 galaxy by a by a foreground galaxy at z = 0.263 and links to a short video SPT0418-47: lensed view to reconstructed view found in YouTube and also at ESO.
The caption says only:

This animation gives the viewer an idea of how astronomers reconstructed the true shape of the SPT0418-47 galaxy, from ALMA observations of the gravitationally lensed view of the galaxy.
SPT0418-47 is gravitationally lensed by a nearby galaxy, appearing in the sky as a near-perfect ring of light, shown at the start of the video. At the end, we see the image of SPT0418-47 that astronomers obtained when they used a new computer modelling technique to reconstruct its real shape and motion of gas.
Credit: ALMA (NRAO/ESO/NAOJ)/Martin Kornmesser (ESO)

Question: So the video begins with the currently observed image and ends with the reconstructed image while keeping the size unchanged. But what do all the intermediate images between these two endpoints represent? Are they intermediate steps in the un-lensing procedure, or are they simply lensed images while computationally ramping down the mass of the foreground lensing galaxy just for fun?


texts: Did the scientists really observed gold and platinum being created from kilonova?

texts: I read that after LIGO and Virgo interferometers detected gravitational waves signal from a kilonova, many ground based telescopes quickly turns their mirrors towards the spot and "sees" gold and platinum ,etc other heavy elements over the next few days. Is it really possible to directly observe heavy elements being formed consider that the event occurs at a distance of around 130 millions light year away?


texts: How much mass does an object in space need to keep a human on its surface?

texts:
Assume there were a roughly spherical object in space like a meteorite or a comet.

If I weighed 80kg on Earth, how much mass would be required for an object in space so that I could stay on its surface without "flying away?" It's not necessary have the same gravity as Earth, but I am wondering what minimal mass the object would require for it to have meaningful gravity for someone to remain on the surface.


texts: How to calculate the limiting magnitude of Hubble?

texts:
I've found various sources on the web stating that the limiting apparent magnitude of the Hubble Space Telescope is about 31 or 31.5. Wikipedia lists that here and it has a cite to a pdf about it.

What I want to know is how is that calculated?

I'm aware of two methods found at the bottom of a table here, but they seems to be general calculations for ground-based telescopes, because if I plug in Hubble's numbers, I get something around 21. The difference between an apparent magnitude of 21 and 31 is actually something like ten thousands times (in terms of brightness).

Also, one key factor missing in their formulas is exposure time. I'm pretty sure the limiting magnitude has something to do with exposure time. Hubble Ultra Deep Field, for example, had an exposure time of about 1 million seconds.

So to sum up, how would we calculate the limiting magnitude of a space-based telescope, in visible light?


texts: What exactly is a "stellar association" in the strict kinematic sense?


texts: What fraction of galaxies in the observable universe have we actually observed?

texts: There are a finite number of currently observable galaxies due to the finite age of the universe and the speed of light. What fraction of these galaxies have we actually observed (by eye, telescope, etc.)?


texts: Are there home-brew photometers for amateur observing of variable stars and are they widely used?

texts:
Comments on Why do these photometric observations of Betelgeuse look “quantized” in 0.1 magnitude steps? suggest that these are visual estimates, mostly (but not all) to 0.1 magnitude resolution.

Are there any designs for simple photodiode-based amateur photometers for stellar brightness measurements?

Just as an illustration of the kind of thing I'm asking about, I'm imagining a pair of binoculars with a 1 mm photodiode suspended by its wires the focal plane of one eyepiece, and a Raspberry Pi recording the photocurrent (with a small amplifier) and audio simultaneously ("I'm now measuring xxx... mark!") The data would be jumpy but presumably would reach a plateau whenever the star fell on the active area of the diode.

Or is this too impractical and not backwards-compatible with existing data?


texts:
I am looking to buy a Celestron 21023 FirstScope 76 Telescope, which is quite inexpensive, and it has good reviews on Amazon.  One thing that many reviewers have said is that the two lenses that it comes with are of low quality, and the suggestion is to toss them and buy a Barlow lens, and perhaps some other more quality lens.

I don't know what a Barlow lens is.

Is this a brand name?  The name of a type of lens?  What distinguishes it from a non-Barlow lens?


texts: Does gravitational lensing cause a black-hole to be the main 'source' of light in a given area?

texts:
Light gets bent around high-gravity sources, such as a black-hole. A source of light, say A, is directly observed in its 'true' position and some of its light is bent around the black-hole, X, which gives a second observation of A, called A'.

Does gravitational lensing cause the amount of emissions (detected as coming from the (point/position/direction) of X) to far exceed anything else in that vicinity?

Does this then make the 'black-hole' by far the brightest, noisiest thing to be observed? (kind of like waving & shouting in a quiet room)

[edit - based on astromax's answer]
I follow what is meant regarding super massive black-holes.
Consider the following scenario: A suitable massive sun on dying contracts into a (small) black-hole. This black-hole will now cause a lensing-effect, but since it is small the amount of emissions so 'lensed' around it is very small and maybe too small to detect? or, it is detected but erroneously ascribed to be a type of sun or other type of emission source?


texts: Stable polar solar orbit with the Earth continuously observable

texts: Is there a stable polar solar orbit such that the Earth is always in view and not eclipsed by the Sun?


texts: Can anyone one show how speeds greater than c cause a paradox on a space time diagram?

texts:
So according to Relativity, time moves at different rates for different observers, and distant observers will actually disagree on the order of events.  Now from what I gather this gets sorted out for objects with relative speeds less than c, but would ultimately cause a paradox for anything moving faster because it would time travel.  However, I don't really see how.  On a spacetime diagram light always moves at a 45° angle, or at least some angle between 0° and 90°, and the only way I see anything effecting the past is if something moves at an angle greater than 90°.

So can someone please give a visual representation that explains the issue?